1. Field of the Invention
This invention is concerned with a process for manufacturing a synthetic zeolite. In particular, it is concerned with a method for the synthesis of nitrogenous zeolites with control of the shape or other characteristics such as size and discreteness of the synthetic microscopic crystals or aggregates thereof formed during crystallization. It is further concerned with a process for manufacturing highly siliceous zeolites such as ZSM-5.
2. Prior Art
The commercial synthesis of zeolites for use as catalysts and as sorbents has become of substantial importance in the last two decades. The zeolites as that term is herein used, refers to those crystalline inorganic structures formed of three-dimensional arrays of silicon and aluminum tetrahedrally coordinated with oxygen in such a manner as to form a network of well defined pores and channels of near-molecular dimensions. The channels usually are filled with water when the zeolite is crystallized, but after dehydration the pore space thus formed is receptive to a variety of organic guest molecules provided these are not too bulky. Because of the regularity of the crystal structure, dehydrated zeolite crystals exhibit a characteristic sorption behavior sometimes referred to as zeolitic sorption. For further background, the reader is referred to "Zeolite Molecular Sieves" by Donald W. Breck, published by John Wiley & Sons, New York, N.Y. (1974), the entire contents of which are herein incorporated by reference.
With certain zeolites, and under appropriate conditions, catalytic transformation of organic molecules occurs when these are brought into contact with the mineral. These reactions generally are believed to occur within the pores of the crystals. Undoubtedly, the most important catalytic application for zeolites is the catalytic cracking of heavy petroleum oils to gasoline and fuel oil, widely practiced in petroleum refineries, but a number of other reactions, too, have been reported. By way of general background, the reader is referred to "Zeolite Chemistry and Catalysis" Jules A. Rabo, A.C.S. Monograph 171, American Chemical Society, Washington, D.C. (1976), the pertinent portions thereof which refer to catalysis and sorption by zeolites being herein incorporated by reference.
Because the sorption or catalytic transformation of substances by zeolites is effected by the channels within the crystals, the orientation of the channel direction with respect to crystal dimensions, the length of the channel, and most probably even the imperfections in the crystal can favorably or unfavorably affect the efficiency or selectivity of a particular specimen of a zeolite for a particular application. Also, certain steps in the manufacture of the zeolite, such as filtration, or in its subsequent compositing with binders or with a matrix, are likely to be affected by the size, the shape, and the surface smoothness of the crystals. Thus, what is herein designated the "microscopic texture" of a particular specimen of synthetic zeolite is an important characteristic of the specimen. By way of illustration, U.S. Pat. No. 4,025,571 to Lago describes a process for converting monohydric alcohols having up to four carbon atoms or their ethers to a mixture of C.sub.2 -C.sub.3 olefins and monocyclic aromatic hydrocarbons with a catalyst, e.g. ZSM-5, having a crystal size of at least 1 micron. U.S. Pat. No. 4,025,572, also to Lago, describes a process for converting lower alcohols, especially methanol, to a mixture rich in C.sub.2 -C.sub.3 olefins and mononuclear aromatics, and Table III therein shows the effect of crystal size on selectivity for ethylene.
The term "microscopic texture" as used herein refers to the character of the individual crystals of zeolite or aggregates thereof as observed with the electron microscope. A synthetic zeolite crystallizes as a fine powder which exhibits an x-ray diffraction pattern characteristic of that particular type of zeolite. Microscopic examination of two different preparations of the same mineral may show, however, that the individual particles of the two powders are very different, the substantial identity of the x-ray diffraction patterns notwithstanding. For example, the two preparations may differ in crystal shape, or in having predominantly large or predominantly small crystals; and while in one preparation the crystals may be largely discrete, or singly twinned, the other may exhibit multiply twinned crystals even to the extent of forming a honeycomb-like or reticulated structure consisting of many small multiply twinned crystals; and, the crystals of the two preparations may vary in smoothness. Different preparations of mordenite, for example, as noted on page 262 of the hereinabove cited reference on "Zeolite Molecular Sieves" by D. W. Breck, demonstrate such dissimilar microscopic appearances. There is no particular consistency among different authors in the use of terms to describe the microscopic texture of a zeolite specimen. The terms and measures used, however, do relate to one or more observable features including the shape, the size, the degree of smoothness, and the degree of discreteness of the ultimate crystals, and it is in this sense that the term "microscopic texture" is used herein. Further, a reference herein to an alteration in microscopic texture, or to a different microscopic texture, in general will refer to a microscopically observable change or difference in at least the shape of the crystals, or the size of the crystals, or the degree of discreteness of the crystals, or the smoothness of the crystals, and to microscopically observable changes or differences in two or more of these features.
The preparation of synthetic zeolites from aqueous, inorganic compositions is known. In general, the zeolites thus prepared, such as Linde A, X and Y zeolites, and synthetic mordenite, have a SiO.sub.2 /Al.sub.2 O.sub.3 ratio not substantially in excess of 10, and that ratio usually is less than 10, having a value such as 2, or 3, or 4.
In recent years work has been done with zeolite forming solutions which contain organic nitrogenous bases. A zeolite formed from such a solution in some cases is similar in crystal structure to that formed from a strictly inorganic solution, and has a SiO.sub.2 /Al.sub.2 O.sub.3 ratio less than 10. This present invention is not concerned with such zeolites. In other cases, however, the organic nitrogenous base appears to act as a template and a new zeolite structure is formed having a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of at least 12. The development of the foregoing art is briefly summarized in the foregoing reference to "Zeolite Molecular Sieves" by Breck at pages 304-312 inclusive, which pages are herein incorporated by reference. The term "nitrogenous zeolites" as used therein by Breck includes all of the above described zeolites regardless of SiO.sub.2 /Al.sub.2 O.sub.3 ratio. For purposes of the present invention, however, it is to be understood that the term refers only to those synthetic zeolites crystallized with a nitrogenous template in the forming solution and that have a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of at least 12. The use of a mixture of tetramethyl ammonium chloride and tetrapropyl ammonium bromide as template for forming ZSM-5 type material is illustrated by Example 5 of U.S. Pat. No. 3,941,871 issued Mar. 2, 1976. Examples of nitrogenous templates described by Breck include tetramethyl ammonium hydroxide, tetraethyl ammonium hydroxide, tetrapropyl ammonium hydroxide, and 1,4 dimethyl-1,4-diazoniacycle (2.2.2.) octane silicate. However, as used herein, the term nitrogenous template refers to any organic basic nitrogen compound or mixture thereof which is required in a specific forming solution to induce crystallization of a particular zeolite with a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of at least 12.
U.S. Pat. No. 4,018,870 to Whittam describes suppression of the formation of unwanted zeolite contaminants during crystallization by adding to the zeolite forming mixture a basic dye, the mol ratio of dye to Al.sub.2 O.sub.3 in the aqueous mixture being less than 0.1 to 1. In some recipes the dye causes a new zeolite to form.
It is an object of this invention to provide a process for manufacturing synthetic nitrogenous zeolites wherein large, easily filtered crystals are formed. It is a further object of this invention to provide a process for manufacturing synthetic nitrogenous zeolites particularly useful as catalyst. It is a further object of this invention to control the microscopic texture of a synthetic nitrogenous zeolite. These and other objects will be apparent to one skilled in the art on reading this entire specification including the claims thereof.